They were old enough to move around (branching) and it was a hot day. So I guess these two were looking for a breeze, any would have been a help.
Many Heron and Egret nests are out in the open at rookeries. They mostly breed in warm climates (here 90-100 degrees). In short these poor birds are baking in the sun often.
Above his wings are dropped, a sign of cooling off.